View source for OS virtualization and emulation on Android

[[Android (operating system)|Android]] devices have the ability to run [[virtual machine]]s or [[Emulator|emulate]] other [[operating system]]s.  It does this either via [[desktop virtualization]], [[platform virtualization]], or emulation via [[compatibility layer]].  

== Desktop virtualization ==
[[Desktop virtualization]] apps are the least resource and space intensive compared to other virtualization types, since the Operating System that is being displayed on the Android device is actually located on another computer on the [[local network]] or elsewhere like on the internet.<ref>{{Cite web|url=https://www.computerworld.com/article/3074236/now-you-can-run-windows-10-on-an-ipad-pro-android.html|title=How to run Windows 10 on an iPad Pro or on Android|last=Evans|first=Jonny|date=2019-01-28|website=Computerworld|language=en|access-date=2019-11-20}}</ref>  Depending upon how the desktop virtualization app works, they use [[Remote Desktop Protocol|RDP]] or can use another protocol of their own.  Most business oriented desktop virtualization apps require specific types of equipment or services in order for the app to fully function.  For example, [[VMware Horizon View|VMware Horizon Client]] requires specific VMware equipment for the app to work.<ref>{{Cite web|url=https://www.vmware.com/products/horizon-apps.html|title=Published Apps {{!}} Horizon Apps|website=VMware|language=en-US|access-date=2019-11-20}}</ref>    

A major downside that desktop virtualization apps have compared to other types of virtualization or emulation technologies is that they require a network connection to the server as stated previously above.

== Platform virtualization ==
[[Platform virtualization]] allows more leverage to the developer as anything that relates to the guest operating system only affects the guest operating system, and not the host operating system.  Due to this it is possible for the guest operating system to be rooted, where as the host operating system remains unrooted.<ref>{{Cite web|url=https://www.xda-developers.com/vmos-lets-you-run-a-virtual-android-environment-on-your-phone/|title=VMOS Lets You Run a Virtual Android Machine on your Phone|date=2019-08-08|website=xda-developers|language=en-US|access-date=2019-11-20}}</ref><ref name=":0">{{Cite web|url=https://www.forbes.com/sites/zakdoffman/2019/10/17/huawei-mate-30-users-hit-by-more-google-mayhem/|title=Huawei Mate 30: New Google Play Option Is Here|last=Doffman|first=Zak|website=Forbes|language=en|access-date=2019-11-20}}</ref>  Due to the nature of platform virtualization and the fact that it can virtualize a rooted guest OS, it has a greater advantage over emulators as it can run applications or utilize packages that require access to the underlying system itself.      

As with all platform virtualization software and applications, they take up a lot of resources of the host in order to do the virtualization.<ref name=":0" />

== Emulation ==

=== Types of emulations ===

==== Emulation of other operating systems ====
Emulation of other operating systems on Android require the usage of some form of compatibility layer, where the compatibility layer must utilize some form of technologies or APIs to run the OS inside of the app container.  This does come with limitations as some emulators utilize PRoot which is a chroot like environment.<ref name=":1">{{Cite web|url=https://play.google.com/store/apps/details?id=exa.lnx.a&hl=en_US|title=AnLinux : Run Linux On Android Without Root Access - Apps on Google Play|website=play.google.com|language=en|access-date=2019-11-21}}</ref><ref name=":2">{{Cite web|url=https://www.xda-developers.com/guide-installing-and-running-a-gnulinux-environment-on-any-android-device/|title=Guide: Installing and Running a GNU/Linux Environment on Any Android Device|first=Braden|last=Farmer|date=2017-01-28|website=xda-developers|access-date=2019-11-21}}</ref>  Unlike terminal emulators that emulate the internal OS with/without any extension package support, it can install actual (for example) Ubuntu packages, as it does not rely too much on the Android system limitations.  However, not all packages and applications can run.

==== Terminal emulation of internal operating system ====
Terminal emulation of the Android device itself is done via either an actual [[Local loopback address|local loopback]] to the device, or an emulation that seems to be a local loopback.  Most of these terminal emulations of the device itself utilize the native terminal [[Toybox]] toolchain's library and functions that come with every android device.<ref>{{Cite web|url=https://developer.android.com/ndk/guides/standalone_toolchain|title=Standalone Toolchains {{!}} Android NDK|website=Android Developers|language=en|access-date=2019-11-20}}</ref><ref name=":3">{{Cite web|url=https://android.googlesource.com/platform/external/toybox/+/refs/heads/master/README#80|title=README - platform/external/toybox - Git at Google|website=android.googlesource.com|access-date=2019-11-20}}</ref>  However due to the fact that the majority of the functions that are readily available without utilizing a compatibility layer, means that Toybox functions can only be utilized.<ref>{{Cite web|url=https://android.googlesource.com/platform/external/toybox/+/refs/tags/android-cts-7.1_r26/README|title=README - platform/external/toybox - Git at Google|website=android.googlesource.com|access-date=2019-11-20}}</ref>  In order for a user to add any other functions or libraries would require the user to root their own device and add the functions or libraries manually.  Some of these terminal emulators allow users to enter Toybox commands implicitly, where those that don't require the usage of "toybox COMMAND".    

===== With enhancements via semi-emulation =====
Some terminal emulators like [[Termux]] allow users to add packages.  This is done via semi-emulating a different environment via the usage of PRoot and/or Toybox in the back-end.<ref name=":3" />  With the semi-emulating, some predefined ported packages can be used and installed without the need of rooting the device, as they do not utilize Android system files, and reside and run in the apps' own data containers and directories.<ref>{{Cite web|url=https://opensource.com/article/18/5/termux|title=Termux turns Android into a Linux development environment|last=comments|first=15 May 2018 Paul Bailey Feed 178up 5|website=Opensource.com|language=en|access-date=2019-11-21}}</ref>  Because Toybox is used mostly in the back-end, it depends on how the app was developed, for Toybox to be accessed from the semi-emulated enhanced terminal.

=== Extensions ===
Many emulators of other OSes allow the use of extensions.  Aside from the use of standard [[SSH]], some emulators allow the use of technologies like [[VNC]] and [[XSDL]].  Via the use of these extensions, the user can have the emulated/virtualized OS running in a desktop-like environment.<ref>{{Cite web|url=https://www.linuxjournal.com/content/userland-turnkey-linux-your-pocket|title=UserLAnd, a Turnkey Linux in Your Pocket {{!}} Linux Journal|website=www.linuxjournal.com|access-date=2019-11-20}}</ref><ref>{{Cite web|url=https://www.techradar.com/how-to/phone-and-communications/mobile-phones/how-to-install-linux-on-an-android-phone-1322579|title=How to install Linux on an Android phone|last=To|first=Mayank Sharma How|website=TechRadar|date=June 2016 |language=en|access-date=2019-11-20}}</ref>

== Limitations ==
The majority of limitations are seen in emulation-based apps rather than the platform virtualization based apps, as the emulation apps must utilize a compatibility layer.  Thus for libraries and packages to work as expected like in a real OS, the compatibility layer must work properly and must be able to provide accurate information.  However, this requires that the compatibility layer or any predefined software it uses (ex. Docker) to have access to many types of system- and device-related information. This can either be done via Toybox or programing language libraries that Android apps can be made from (e.g., [[Java (programming language)|Java]], [[C Sharp (programming language)|C#]]).  However, starting with Android 8, many new security restrictions have been placed on apps; due to these restrictions, some API libraries/classes are no longer available or have been replaced by more restrictive API libraries/classes.<ref>{{Cite web|url=https://source.android.com/setup/start/p-release-notes|title=Android 9 Release Notes|website=Android Open Source Project|language=en|access-date=2019-11-20}}</ref>     

One of the known limitations that was implemented is that apps can only natively access only their own sandbox root directory or sub-directories on an SD card.  Apps wanting to access files or folders outside that on the SD card, are required to use Android's own system file selector.<ref>{{Cite web|url=https://source.android.com/setup/start/android-10-release|title=Android 10 Release Notes|website=Android Open Source Project|language=en|access-date=2019-11-20}}</ref>  Due to this limitation, guest OSes running on, emulation, platform virtualization, or compatibility apps are no longer allowed to access files outside of their own SD card Sandbox.       

The majority of these limitations can be bypassed by rooting the Android device. However, utilizing Linux via utilizing an emulator or virtualization app would be redundant at that point as one could turn the device itself into a Linux environment.<ref>{{Cite web|url=https://developers.redhat.com/blog/2017/03/16/installing-linux-on-an-android-phone/|title=Installing Linux on an Android Phone|date=2017-03-16|website=Red Hat Developer|language=en-US|access-date=2019-11-20}}</ref>         

=== OS internal security enhancements ===
More modern versions of Android have been seen having more internal system security enhancements, such that some system folders and files are no longer accessible or even viewable for that directory's existence.  Versions of Android before 9 allowed terminal apps to view Android's root "/" directory (although not allowing modifications or execution).  <ref>{{Cite web|url=https://source.android.com/devices/bootloader/system-as-root|title=Partition Layout|website=Android Open Source Project|language=en|access-date=2019-11-20|archive-date=2019-12-24|archive-url=https://web.archive.org/web/20191224042021/https://source.android.com/devices/bootloader/system-as-root|url-status=dead}}</ref><ref>{{Cite web|url=https://source.android.com/setup/start/p-release-notes|title=Android 9 Release Notes|website=Android Open Source Project|language=en|access-date=2019-11-20}}</ref>  

==== SELinux ====
Android utilizes [[SELinux]]; due to this, all system configuration files are not editable and some are not even readable.<ref>{{Cite web|url=https://source.android.com/security/selinux|title=Security-Enhanced Linux in Android|website=Android Open Source Project|language=en|access-date=2019-11-20}}</ref>  Thus, apps can only utilize Android API's to learn about the device's or system's configuration or settings.  However, not all information is available to apps; due to this, many OS compatibility layers or emulation apps are not able to run every package or application.  

Here are some known applications and packages that are unable to run:  

* [[Udisks|Udisks2]] requires udev which does not work well in most compatibility layer environments, as stated in the sub-sub-section below.<ref>{{Cite web|url=https://github.com/CypherpunkArmory/UserLAnd/issues/992|title=Apt-Get Install failing - Udisks2 Permission Denied error leads to lots of DPKG error · Issue #992 · CypherpunkArmory/UserLAnd|website=GitHub|language=en|access-date=2019-11-20}}</ref><ref>{{Cite web|url=https://www.andropalac.com/install-parrot-on-android/|title=[Full Tutorial💻] Install Parrot OS On Android|date=2019-10-09|website=AndroPalac.com|language=en-US|access-date=2019-11-20}}</ref>
* [[Snort (software)|Snort]]
* [[Open vSwitch]]
* [[mininet]] (requires open vSwitch to run)

Here are some commands that do not work, due to SELinux:  

* ip link show <ref>{{Cite web|url=https://www.researchgate.net/post/How_can_I_change_MAC_address_on_Android_devices_without_Root2|title=How can I change MAC address on Android devices without Root?|website=ResearchGate|language=en|access-date=2019-11-20}}</ref>

====ADB, PRoot, $HOME and $PREFIX usage====
Because all Android apps run in a non-privileged container space, the virtualization or emulation apps must utilize their own app space to do the virtualization or emulation.<ref>{{Cite web|url=https://source.android.com/setup/start/p-release-notes|title=Android 9 Release Notes|website=Android Open Source Project|language=en|access-date=2019-11-20}}</ref>  This can be done either by ADB, PRroot, or the usage of $HOME and $PREFIX, however majority of these three are utilized by emulation apps as platform virtualization apps usually use different methods.  Each of these is used for a different purpose.  ADB is used for the emulation of a terminal-like environment for the entire Android system, but usually as an unprivileged user.  $HOME and $PREFIX can be using $HOME as "~" and $PREFIX for ".".  However, $HOME can not be set outside of the "/data/apps data space" directory on the Android file system, nor can $PREFIX be moved to another place like the SD Card.<ref name=":4">{{Cite web|url=https://wiki.termux.com/wiki/Differences_from_Linux|title=Differences from Linux - Termux Wiki|website=wiki.termux.com|access-date=2019-11-20}}</ref>  

For emulators or compatibility layers to emulate or virtualize a Linux-based OS on Android, [[PRoot]] must be used.<ref name=":1" />  However, when PRoot is used, the environment inside of Proot can act like a [[Chroot]] environment, and/or a separate mounted environment.<ref>{{Cite web|url=http://manpages.ubuntu.com/manpages/bionic/man1/proot.1.html|title=Ubuntu Manpage: PRoot - chroot, mount --bind, and binfmt_misc without privilege/setup|website=manpages.ubuntu.com|access-date=2019-11-20}}</ref><ref>{{Cite web|url=https://packages.debian.org/sid/proot|title=Debian -- Details of package proot in sid|website=packages.debian.org|access-date=2019-11-20}}</ref>  In this scenario (especially in the case of compatibility layer and emulator for Guest OSes), there are packages and applications that can not run as they were never designed to run inside a chroot like environment.<ref name=":2" />  Some of these are well known, prominent packages, applications, or commands, including the following:       

* [[Systemd]]     
* [[Libudev]] (Works alongside Systemd).<ref>{{Cite web|url=https://wiki.archlinux.org/index.php/Udev|title=udev - ArchWiki|website=wiki.archlinux.org|access-date=2019-11-20}}</ref>
* [[Init]]     
* [[Mutter (software)]] relies upon systemd 

===== Alternatives =====
Due to the issue of SeLinux and PRoot, alternatives must be used and not all of the unusable packages or applications have alternatives.  Being an alternative, they don't necessarily have all the functions of the original packages or applications.  Some of these alternatives include:  

* [[Eudev]]
* [[Elogind]]
* [[Metacity]] (Predecessor of Mutter)

===Android itself including a modified Linux kernel===
Because Android utilizes a modified Linux kernel, not all configuration or system files exist or are the same in Android.<ref>{{Cite web|url=https://source.android.com/security/overview/kernel-security|title=System and kernel security|website=Android Open Source Project|language=en|access-date=2019-11-21}}</ref><ref>{{Cite web|url=https://unix.stackexchange.com/questions/25463/does-android-really-use-the-same-kernel-as-linux|title=Does Android really use the same kernel as Linux?|website=Unix & Linux Stack Exchange|access-date=2019-11-21}}</ref>  Because of this, emulation apps either need to provide either their own version of the configuration or system files in the application's data space, use a pre-modified distribution of ported Linux libraries that do not require the usage of the missing configuration or system files, or not support the usage of those packages.  Some well known configuration and system files that are not in Android include:    

* /etc/resolv.conf<ref name=":4" />

== See also ==
* [[Comparison of OS emulation or virtualization apps on Android]]
* [[Virtualization]]

== References ==
<references />

[[Category:Android (operating system) software]]
[[Category:Virtualization software]]
[[Category:Emulation software]]